Study On The Morphologies And Growth Mechanism Of Zn Crystals On Ionic Liquid Surfaces

In recent years,owing to the widespread application of low-dimensional nanocrystals in semiconductor and other industries,more and more attention has been paid to the nanocrystals,in which one-or two-dimensions of the nanocrystals is in the range of 1-100 nm.In many experiments so far,anisotropic crystal material surfaces are generally used as the substrates when people hope to fabricate one-and two-dimensional nanocrystals.The main reason is that the crystal surfaces are directional and periodic,which is mactched with the characteristics of the nanocrystals and helpful for the growth of the low-dimensional nanocrystals.Therefore,the fabrication for the nanocrystals on liquid substrates is rarely reported.In this thesis,we report the research results about the low-dimensional Zinc(Zn)nanocrystals,which were prepared successfully on the ionic liquid surfaces by thermal evaporation method.The influences of the liquid substrate characteristics and substrate temperature on the morphologies of the low-dimensional Zn nanoncrystals are studied,the relevant growth mechanism of the nanocrystals are also analyzed and investigated.The experimental results indicated that,under the identical substrate temperatures(Ts = 20 ?),nominal deposition rates(f = 0.01 nm/s)and nominal deposition thickness(d = 8.0 nm),the low-dimensional Zn nanocrystals(nanowires,nanorods and diamond disk etc)may be prepared on both the silicone oil and ionic liquid surfaces successfully.However,the average size of Zn crystal nanorods grown on the ionic liquids are several times larger than that of the nanorods grown on silicone oil surfaces.We believe that the physical properties of the liquid substrates,such as the viscosity coefficient,etc.are the crucial factor which results in the obvious different morphologies of the Zn nanocrystals.In addition,if the nominal deposition rates(f = 0.01 nm/s)and nominal deposition thickness(d = 8.0 nm)were fixed and two different substrate temperatures were used,i.e.Ts1 == 20 ? and Ts2 = 28 ?,for the samples deposited on the ionic liquid surfaces,we found that the surfaces of the Zn crystal nanorods,which usually connected with the bulks crystals,become rougher as the substrate temperature increases.The statistical results indicate that,the higher the substrate temperature,the smaller the width of the Zn crystal.We suggest that the viscosity coefficient of the ionic liquids decreases as the substrate temperature goes up,which may increase the the diffusion coefficient and enlarge the average diffusion length of the Zn atoms on the liquid surfaces.Finally,it will result in the formation of the large size Zn nanocrystals on the ionic liquid surfaces.Based on the experimental observation and the previous research results,the growth mechanism of the low-dimensional nanocrystals grown on the isotropic liquid surfaces may be summarized as follows:Firstly,due to the isotropic and quasi-free standing characteristics of liquid substrates,the deposited Zn atoms possess considerable diffusion coefficient,and thus they can diffuse freely and randomly on the liquid surface.The Zn atoms may aggregate and form clusters or nanocrystals when they meet;Secondly,because of the hexagonal-close-pack(hcp)crystal microsturcture of the Zn crystals,there are remarkable surface energy differences for different crystal planes.Therefore,the Zn nanocrystals possess preferential growth directions during the growth,which results in the anisotropic crystal growth manner.It is obvious that the quasi-free standingcharacteristics of the liquid surfaces is much more beneficial to the growth of the one-and two-dimensional Zn nanocrystals.On the other hand,our experiments show that the preferential growth behavior does not exhibit absolute advantage.It is noted that there are several sub-preferential growth directions for the Zn crystals,and various defects may be produced during crystal growth,which may result in the abundant morphologies of the Zn crystals on the liquid substrates.The phenomenon above has been certified by a series of SEM images.From the SEM images,we can see that crystal nanowires,nanorods with break angles,or one-,two-and three-dimensional nanocrystals may appear simultaneously on one piece of Zn sample.The chapters of this thesis are arranged as follows:Chapter 1:The preparation method,growing conditions and physical properties of the general crystals are mainly reviewed.The current research status and progress of the fabrication of nanosystems on liquid substrates are introduced.Chapter 2:The preparation method of Zn nanocrystals grown on ionic liquid surfaces are detailed.Chapter 3:The experimental results for preparing Zn nanocrystals on ionic liquid surfaces are given.The influences of the substrate characteristics and substrate temperatures on the morphology of the Zn crystals are introduced.And its growth mechanism has been proposed.Chapter 4:The main experimental and theoretical results of this thesis are concluded.Some suggestions for the future research are proposed.